I put down reasonably priced; if you already own carving tools you won't need to spend too much. If you don't, you can make stencils out of regular paper instead, which will be super cheap!
The project itself doesn't take too long, but the paint usually has to dry for four hours.
Wash your t-shirt before you stamp it.
Cave out the text stamps you want from linoleum blocks.
Remember to do the text/image mirrored so it will be the right way when stamped! I failed to do so on my first try, but I kinda like it anyway.
PS: I made stamps since I wanted to be able to use them again. You can cut out stencils from regular paper instead, which will be cheaper (but don't do those mirrored!).